Thermal and electrochemical process for metal production

ABSTRACT

A system for purification of high value metals comprises an electrolytic cell in which an anode formed of a composite of a metal oxide of the metal of interest with carbon is electrochemically reduced in a molten salt electrolyte.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/496,981, Filed Aug. 20, 2003.

FIELD OF THE INVENTION

The present invention relates to the production of metals. The inventionhas particular utility in connection with the production of titanium andwill be described in connection with such utility, although otherutilities are contemplated, e.g., production of other high valuemulti-valence and high (2 or more) valence metals, in particularrefractory metals such as chromium, hafnium, molybdenum, niobium,tantalum, tungsten, vanadium and zirconium which are given as exemplary.

BACKGROUND OF THE INVENTION

The properties of titanium have long been recognized as a light, strong,and corrosion resistant metal, which has lead to many differentapproaches over the past few decades to extract titanium from its ore.These methods were summarized by Henrie [1]. Despite the many methodsinvestigated to produce titanium, the only methods currently utilizedcommercially are the Kroll and Hunter processes [2, 3]. These processesutilize titanium tetrachloride (TiCl₄) which is produced from thecarbo-chlorination of a refined titanium dioxide (TiO₂) according to thereaction:TiO₂(s)+2Cl₂(g)+2C(s)→TiCl₄(g)+2CO(g).In the Kroll process [2] TiCl₄ is reduced with molten magnesium at ≈800°C. in an atmosphere of argon. This produces metallic titanium as aspongy mass according to the reaction:2Mg(l)+TiCl₄(g)→Ti(s)+2MgCl₂(l)from which the excess Mg and MgCl₂ is removed by volatilization, undervacuum at ≈1000° C. The MgCl₂ is then separated and recycledelectrolytically to produce Mg as the reductant to further reduce theTiCl₄. In the Hunter process [3,4] sodium is used as a reductantaccording to the reaction:4Na(l)+TiCl₄(g)→Ti(s)+4NaCl(l)The titanium produced by either the Kroll or Hunter processes must notonly be separated from the reductant halide by vacuum distillationand/or leaching in acidified solution to free the titanium sponge forfurther processing to useful titanium forms, but also require therecycling of the reductant by electrolysis. Because of these multiplesteps the resultant titanium is quite expensive which limits its use tocost insensitive applications.

The US Bureau of Mines performed extensive additional investigations[1,5-8] to improve the Kroll and Hunter processes. Many other processeshave been investigated that include plasma techniques [9-13], moltenchloride salt electrolytic processes [14], molten fluoride methods [15],the Goldschmidt approach [16], and alkali metal-calcium techniques [17].Other processes investigated without measurable success have includedaluminum, magnesium, carbothermic and carbo-nitrothermic reduction ofTiO₂ and plasma reduction of TiCl₄[18]. Direct reduction of TiO₂ orTiCl₄ using mechanochemical processing of ball milling with appropriatereductants of Mg or calcium hydride (CaH₂) also have been investigated[19] without measurable success. Kroll, who is considered as the fatherof the titanium industry [20] predicted that titanium will be madecompetitively by fusion electrolysis but to date, this has not beenrealized.

An electrolytic process has been reported [21] that utilizes TiO₂ as acathode and carbon or graphite as the anode in a calcium chlorideelectrolyte operated at 900° C. By this process, calcium is deposited onthe TiO₂ cathode, which reduces the TiO₂ to titanium and calcium oxide.However, this process is limited by diffusion of calcium into the TiO₂cathode and the build-up of calcium oxide in the cell, which limitsoperating time to remove the calcium oxide or replacement of theelectrolyte. Also the TiO₂ cathode is not fully reduced which leavescontamination of TiO₂ or reduced oxides such as TiO, mixed oxides suchas calcium titanate as well as titanium carbide being formed on thesurface of the cathode thus also contaminating the titanium. Thus,current TiO₂ cathode electrolytic processes are no more commerciallyviable than earlier electrolytic processes.

SUMMARY OF THE INVENTION

The instant invention is a combination of a thermal and anelectrochemical process, which utilizes a carbon or composite anodecontaining a metal oxide of a metal of interest, as a feed electrode. Asused herein the term “carbon” is meant to include carbon in any of itsseveral crystalline forms including, for example, graphite. For example,for producing purified titanium, the feed should comprise TiO₂ which maybe high purity, rutile, synthetic rutile, illuminate or other source oftitanium, mixed with a source of carbon and pressed together with orwithout a binder that also may be a source of carbon on pyrolysis toform a TiO₂—C composite green electrode or billet. The TiO₂—C compositebillet is then heated, in the absence of air to avoid oxidation of thecarbon component, to a temperature sufficient to reduce the plus fourvalence of the titanium in the TiO₂ to a lower valence. The temperatureof heating and time at temperature will determine the reduced oxidestoichiometry of the titanium oxide which may be expressed asTi_(x)O_(y) where the ratio of y/x can be 0 to equal or less than 2 andy balances the valence charge of the titanium species. Some examples ofreduced titanium oxide compounds include TiO, Ti₂O₃, Ti₃O₅, and Ti₄O₇and mixtures thereof. Sufficient residual carbon needs to remain afterthe thermal reduction step or can be added separately tostoichiometrically react with the reduced titanium oxide toelectrochemically produce titanium at the cathode and CO₂ and/or CO atthe anode. The reduced titanium state oxide composite anode overallgeneral reactions are:

${{{Ti}_{x}O_{y}} + {\left( \frac{y + n}{z} \right)C}} = {{x{Ti}} + {n{CO}} + {\left( \frac{y - n}{z} \right){CO}_{2}}}$

-   -   at the anode:

${{{Ti}_{x}O_{y}} + {\left( \frac{y + n}{z} \right)C}} = {{x{Ti}}^{{+ 2}{y/x}} + {n{CO}} + {\left( \frac{y - n}{2} \right){CO}_{2}} + {z\; y\; e^{-}}}$

-   -   where 2y/x is the oxide state of the titanium in the        electrolyte,    -   at the cathode:        xTi^(+2y/x) +zye ⁻ =xTi

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will be seen bythe following detailed description, taken in conjunction with theaccompanying drawings wherein:

FIG. 1 is a diagrammatic illustration schematically illustrating anelectrochemical reaction according to the present invention;

FIG. 2 a is a diagrammatic illustration of electrochemical process ofthe present invention;

FIG. 2 b is a diagrammatic illustration of an electrochemical cell andprocess in accordance with the present invention;

FIG. 3 is a view similar to FIG. 2 b providing further details of anelectrochemical cell in accordance with the present invention;

FIG. 4 is a perspective view showing details of an electrode inaccordance with the present invention;

FIG. 5 is a graph illustrating surface resistivity of a titanium oxidecarbon anode over time.

DETAILED DESCRIPTION OF THE INVENTION

The present invention employs a novel electrochemical system forproducing titanium and other metals by a combination of thermal andelectrochemical processes from a novel metal oxide-carbon compositeanode. More particularly, the present invention produces purifiedtitanium or other metal powders by a thermal/electroduction compositeanode process using a metal oxide-carbon anode in a molten saltelectrolyte.

Heretofore the electrolysis of titanium oxide (TiO₂) has not beensuccessful because TiO₂ has little to no solubility in molten saltelectrolytes which is also true of other titanium compounds. Titaniumtetrachloride (TiCl₄) is a covalent compound that has limited solubilityin fused salts and does not readily form complexes with other inorganicsalts. It also is highly volatile and is quickly lost from most fusedsalts. However, since titanium is multivalent, it has been shown thatTiCl₄ could be reduced to lower valent ionic species of Ti⁺³ and Ti⁺²,which do exhibit some solubility in some molten salts. However, becauseof secondary reversibility reactions, which lead to loss in currentefficiency and poor quality of metal, heretofore no practical processhas evolved for electrowinning titanium from a TiCl₄ feed.Investigations of separating the anolyte and catholyte to avoidalternating oxidation and reduction with low current efficiency have notproven successful on a commercial scale.

Since titanium+3 (corresponding to y/x of 1.5) and titanium+2(corresponding to y/x of 1.0) are ionic species, it should be possibleto deposit titanium at the cathode, i.e. according to the reactions:Ti⁺³+3e=Ti ⁰ or Ti⁺³ +e=Ti ⁺² and Ti⁺²+2e=Ti ⁰from a molten salt electrolyte. However, such reactions have not beendemonstrated commercially since heretofore there has not beendemonstrated an acceptable process to continuously supply Ti^(+2y/x) orlower valence ions where y/x is less than 2 to a molten saltelectrolyte. The present invention in one aspect provides a metaloxide/carbon composite anode containing Ti_(x)O_(y) in which a highvalence metal such as Ti⁺⁴, is thermally reduced to a valence less than+4, and is used to provide a continuous supply of reduced titanium ionsto a molten salt electrolyte. The oxygen combines with the carbon in theanode to produce CO₂ and/or CO gas. Any excess carbon in the anodefloats to the top of the molten salt electrolyte where it periodicallycan be skinned if necessary and does not interfere with the continuouselectrolysis process.

It is well established that thermal reduction is much more economicalthan electrochemical reduction. Therefore reducing TiO₂ thermally ismore economical than electrolytically reducing in a composite anode ofTiO₂-carbon. If TiO₂ is heated with carbon, carbo-thermic reduction willproceed based on the thermodynamic prediction and kinetics of thereactants. For example it has been found when the proper proportions ofTiO₂ and carbon are heated to various temperatures, reduced oxides areproduced. An example reaction is 2TiO₂+C=Ti₂O₃+CO. The Ti₂O₃ in whichthe titanium is in a +3 valence state can be produced over thetemperature range of 1250-1700° C. Since the product is a solid Ti₂O₃and gaseous CO if the pressure is reduced the kinetics of the reactionsis enhanced.

It is also possible to produce the suboxide TiO according to thereactions TiO₂+C=TiO+CO or Ti₂O₃+C=2TiO+CO. Either reaction will beenhanced at reduced pressure. Titanium in TiO is in the +2 valencestate. A competing reaction is TiO₂+3C=TiC+2CO or Ti₂O₃+5C=2TiC+3CO.When the suboxide is used as a feed for the composite anode, the lowestvalence is the most desirable. Thus it is desirable to prevent TiCforming in which the titanium is in a +4 state. It has been found thatTiO can be produced at a reaction temperature above 1700° C. if thepressure is reduced to 0.01 atmosphere or lower. If the pressure is ashigh as 0.1 atmosphere a reaction temperature above 1800° C. is requiredto produce TiO free of TiC. At atmospheric pressure a reactiontemperature above 2000° C. is required to produce TiO free of TiC.

In addition to producing titanium from a composite anode consisting of areduced titanium oxide and a carbon source referred to as a compositeanode it is also possible to electrowin titanium from other titaniumcompounds that are not oxides. These compounds include titanium nitride(TiN). Titanium nitride is a conductor and does not require anyconductive phase such as carbon with titanium suboxides. TiN can beproduced by reacting TiO₂+2C+N=TiN+2CO. The TiN is pressed and sinteredin a nitrogen atmosphere to produce a solid of TiN. The TiN can then beutilized as an anode in a fused salt to electrowin/deposit titanium atthe cathode and nitrogen gas will be evolved at the anode.

Another compound is titanium carbide (TiC). Titanium carbide is producedby the reaction of TiO₂+2C=TiC+2CO. The TiC is a conductor and when TiCparticles are pressed and sintered to a solid, the solid can serve as ananode. When using TiC as the anode a separator or diaphragm shouldseparate the cathode and anode compartments. Titanium ions will beelectrolytically dissolved from the anode and reduced to titanium metalat the cathode. The released carbon will be in solid form and must beaccounted for in an overall materials balance. To account for the carbonthe anode can be depolarized with oxygen wherein the oxygen will reactwith the carbon to form gaseous CO₂ and/or CO. Thus oxygen gas would bepassed over the anode to react with the carbon, but since titanium is sosensitive to oxygen the cathode should be separated from the anode witha diaphragm to prevent the oxygen from contacting the depositedtitanium.

It is taught in WO09964638, U.S. Pat. No. 6,663,763B2, WO 02/066711 A1,WO 02/083993 A1 and WO03/002 785 A1, that TiO₂ can serve as a cathode ina calcium chloride fused salt wherein the TiO₂ is reduced to titaniummetal with oxygen given off at the anode using an inert anode or CO₂/COusing a carbon/graphite anode. Those teachings do not consider reducedor suboxides of titanium which require less electrochemical energy toproduce titanium metal than required to reduce TiO₂. Thus the reducedoxides of Ti₂O₃ or TiO can serve as cathodes and be electrochemicallyreduced in molten calcium chloride or other molten salt electrolytes.

Heretofore, there has not been an electrochemical system to producetitanium similar to electrowinning aluminum in which alumina (Al₂O₃) issoluble in molten cryolite (NaAlF₄) which under electrolysis producesaluminum metal with CO₂/CO being given off at a carbon anode, becausethere has not been identified a molten salt composition that willdissolve TiO₂. There is no known molten salt compound or combination ofcompounds that will dissolve TiO₂. However, there are molten saltcompositions that will dissolve the reduced suboxide TiO which is anionic compound that is very electrically conductive. For example TiO issoluble in molten calcium chloride mixed alkali and alkaline earthchlorides as well as fluorides or mixed chlorides and fluorides. ThusTiO can be dissolved in CaCl₂ or other salt mixture, and using acarbon/graphite anode electrolyzed to produce titanium at the cathodeand CO₂/CO at the anode or oxygen using an inert anode. Since titaniumis sensitive to oxygen a separator or diaphragm should be used betweenthe anode and cathode.

It is well known that the higher the temperature of a solvent thegreater the solubility of the solute. In this case the higher the moltensalt temperature the greater the solubility of a titanium suboxide suchas TiO or Ti₂O₃. In the previous discussions the operating salttemperatures are below that of the melting point of titanium and thustitanium is deposited as a solid in a particulate morphology. As in thecase of electrowinning aluminum in which aluminum oxide is soluble incryolite at over 900° C., the aluminum is in a molten state and thusmore easily separated from the molten salt/cryolite. In order to achievethe same advantage with titanium, the molten salt operating temperatureshould be above the melting point of titanium or about 1670° C. Moltensalts that have high melting temperatures that will not readily vaporizeat 1670° C. or slightly above include calcium fluoride (CaF₂) 1360° C.,and barium fluoride BaF₂ 1280° C. It was found the titanium suboxidesand particularly TiO is quite soluble in CaF₂ at temperatures above1670° C. Thus titanium is readily electrowon from its suboxidesdissolved in CaF₂ or other salts above 1670° C. using a carbon/graphiteanode that produces CO and CO₂ on electrolysis or an oxygen stable anodethat produces oxygen on electrolysis. The titanium produced above 1670°C. is in a molten state and thus readily separatable from the moltensalt whose density is less than 3.0 g/cc at the operating temperatureand titanium is approximately 4.0 g/cc at the operating temperature thuscausing the titanium to sink for easy separation.

Referring to FIG. 1, there is illustrated schematically the formation ofa metal oxide-carbon composite anode in accordance with the presentinvention. Titanium oxide in a particle size of 0.001-1000 microns,preferably 0.01-500 microns, more preferably 0.1 to 10 microns, is mixedwith carbon flakes of average particle size 0.001-1000 microns,preferably 0.01-100 microns, more preferably 0.01 to 1 microns, in aweight ratio of TiO₂ to carbon of 7:1 to 4:1 using a ball mill mixer.The TiO₂ powder and carbon flakes were mixed dry, or optionally with abinder, in a ball mill mixer for 4-24 hours. The resulting TiO₂powder/carbon flake mix was pressed in a steel die to form amechanically stable green electrode or billet. The billet was thenplaced in an oven, and heated in the absence of air to 1000 to 2200° C.,preferably about 1100° C. to 1800° C., for 0.1 to 100 hours, preferablyabout two hours, to form a titanium suboxide/carbon composite electrode.

Referring to FIGS. 2 a and 2 b, the titanium oxide/carbon compositeelectrode 20 made as above described is employed as an anode in anelectrochemical cell 22 with a conventional metallic, e.g., steelelectrode 24, and an alkali metal molten salt electrolyte 26.

The composition of the molten salt electrolyte 26 used in the cell 22has an effect on the titanium produced at the cathode. The electrolyteshould comprise a strong Lewis acid formulation such as NaAlCl₄, whichmelts as low as 150° C., optionally containing fluoride additions suchas an alkali fluoride and/or potassium titanium fluoride with thereduced state Ti_(x)O_(y)—C anode. Other useful electrolyte compositionsinclude binary, tertiary, and quarterary alkali and alkaline earthchlorides, fluorides and mixed chloride-fluorides with melting pointtemperatures in the 300-900° C. range. For producing titanium preferredelectrolytes include NaCl—CaCl₂—KCl in a mole ratio of 50:50:20;NaCl—LiCl—KCl in a mole ratio of 20:60:40; AlCl₃—NaCl—NaF in a moleratio of 70:30:20 L:Cl—KCl eutectic with 20 wt % NaF, eutectic ofLiF—KF, etc. Moreover, the polarizing strength of the cation willdirectly affect the electroreduction of electrocrystallization totitanium. And, the small highly ionic strength and steric effect ofe.g., a lithium ion in the electrolyte enhances the polarizing strengthat the cathode and thus the electroreduction of titanium. Other suchhighly ionic ions can aid in stabilizing the Ti⁺³ and/or Ti⁺² ions inthe molten salt electrolyte as well as their electroreduction at thecathode.

To avoid disproportionation during the electrolysis between titanium inthe metallic state, that is electrowon titanium, and higher titaniumions such as Ti⁺³, it is preferable to have only Ti⁺² ions in solutionwhich as they are reduced to the metal are replaced with other Ti⁺² ionsfrom the anode thus requiring TiO in the anode. Thus desirably the fusedsalt initially contains Ti⁺² ions which desirably is in theconcentration range of 1/2 to 20%, more desirably in range of 1 to 10%and most desirably in the range of 2 to 8%.

The anion also can have an influence on the steric and solvent effect ofthe titanium species, which also influences the titanium deposit at thecathode. For example, the Ti—F bond is stronger than the Ti—Cl bond,which brings about an increase in the activity of the titanium ions inthe molten salt electrolyte and consequently the morphology of thetitanium deposited at the cathode. The anion and the titanium ioncomplex effects the number of crystallization centers available on thecathode and thus the morphology of the titanium cathode deposit. Thecomplex TiF₆ ⁻³ and the TiF₆ ⁻² anion is known and can be directlyreduced to titanium. Mixed anions are also known, such asTiF_(6-N)Cl_(N) ⁻³. A strong Lewis acid thus stabilizes and increasesthe activity of the titanium ion. While not wishing to be bound bytheory, it is believed that the reactions proceed as follows:TiF₆ ⁻³+3e=Ti⁰+6F⁻and at the anode Ti⁺³ ions are released from the composite anode toproduce the TiF₆ ⁻³. Thus titanium is directly reduced from the +3valence to the metal. Because titanium is multivalent it is alsopossible that Ti⁺³ is reduced to Ti⁺² and then to the metal Ti⁰.However, as stated above, if all titanium ions in solution are in the +2valence then the reduction is Ti⁺²+2e=Ti⁰.

Based on this analysis alkali fluorides may be regarded as stabilizingagents in chloride molten salt electrolytes. Thus the ratio of F/Cland/or Ti/F will have an effect on the electroreduction of titanium.Indeed it has been demonstrated that all chloride molten saltelectrolytes produce small and/or dendritic deposits of titanium. Asfluorides are added to the molten salt electrolyte the morphology of thedeposit changes to larger and coherent particulate deposits. As theelectrolyte changes to primarily or all fluoride, the titanium depositsbecome flaky to a fully adherent film. The major morphology changebegins at a F/Cl ratio of approximately 0.1 and solid films becomepossible at a ratio of approximately 1.0.

The morphology and size of the titanium deposit is also influenced bythe current density of the cathode. The higher the current density thesmaller the particle size. Typical cathode current densities are in the0.05 to 5 ampheres/cm² range. The more desirable cathode currentdensities are in the 0.1 to 2.0 ampheres/cm² range, and the preferredcathode current densities are in the 0.25 to 1 ampheres/cm² range,depending on the morphology of the titanium desired at the cathode. Italso has been found that very high current densities can be used at thecathode under high mass flow of the electrolyte and the use of thecomposite anode. By moving the electrolyte over the cathode surface viagas bubbling or pumping at a fast rate it is possible toelectrolytically produce titanium particularate up to cathode currentdensities of 125 amps/cm².

It also has been found that pulsing the current affects the morphology,particle size and cathodic efficiency. The current can be pulsed to onand off sequences in various wave forms such as square, sinusoidal, etc.as well as periodically alternating the polarity. It was found pulsingthe current produced more coherent deposits and larger particles as wellas solid films on the cathode. It was also found periodically reversingthe polarity between two composite electrodes produced titanium withinthe electrode. That is the Ti_(x)O_(y) in the electrode was reduced totitanium, which remained as a solid agglomerate of titanium particles inthe same form of the original composite electrode.

A bench scale electrolytic cell for producing titanium in accordancewith the present invention is illustrated in FIG. 3. The cell 30comprises a cylindrically shaped steel walled vessel 32 having afunnel-shaped bottom closed by a valve 36. The vessel walls 32 arewrapped in a resistance heater (not shown) which in turn is covered bythermal insulation 40. A porous basket 42 formed of carbon fiber mesh issuspended within container 30 and is connected via an anode connector 44to the plus side of the DC current source. Wall 32 of the steel vesselis connected via a conductor 46 to the negative side of a DC currentsource. Basket 42 is loaded with pellets or discs 48 of titaniumsuboxide-carbon flake anode material made as above described. The cellis filled with a molten salt electrode (60:LiCl-40KCl), the cell issealed with a top 50, swept with argon purge to remove air, and avoltage of 3.0V applied which resulted in precipitation of dendritictitanium sponge particles. The titanium sponge particles were thenremoved via valve 36, separated from the electrolyte, and found to havea purity of 99.9%.

It is possible to deposit other metals similarly. For example, by usinga composite anode which includes other metal oxides in addition to theTi_(x)O_(y), it is possible to produce an alloy of titanium. Forexample, an alloy of Ti—Al—V can be produced by mixing aluminum oxide,vanadium oxide and TiO₂ with carbon to form the anode whereby to producealloy particulate or solid films of Ti—Al—V. The E₀ and current densityshould be adjusted to deposit precise composition alloy particles. Othermetals or alloys can be produced by incorporating other metal oxides inthe anode in accordance with the present invention.

From a practical commercial standpoint of producing titanium particulatein which the particulate can be used directly in powdered metallurgicalfabrication or consolidated into billets for subsequent rolling intosheet, forging, etc. it is desirable to produce the particulate at aslow a cost as possible. High mass transfer and high current density thatproduces particle sizes that are desirable for commercial applicationscan be achieved in a cell configuration such as shown in FIG. 4.

In this case the anode container can be a porous carbon-carbon or otheranodic container in which Ti_(x)O_(y)—C anode segments 60 are placed,and the structural container can be the cathode and/or a cathode 62placed inside the structural container (not shown). Preferably thecontainer is insulated to maintain heat in the molten salt electrolyteto achieve thermal neutrality with the IR/joule heating of theelectrolyte at high current densities. Also if desired the molten saltelectrolyte could be pumped through cyclone systems and filters tocontinuously collect the titanium particulate as it is being produced.Commercial pumping systems are readily available to handle pumpingmolten salt electrolytes such as used in the aluminum and mass solderingindustries to pump molten metals. Molten salt electrolytes that aredesirable for high mass transfer cell designs of which FIG. 4 is justone example, include strong Lewis acid compositions such as NaAlCl₄ andfluoride compositions, and fluoride or chloride alkali and alkalineearth metal salts and mixtures thereof. Utilizing a high mass transfercell design in which the molten salt electrolyte is pumped over thecathode surface, with high stirring rates and/or ultrasonics to agitatethe molten salt electrolyte or the cathode itself coupled with a reducedvalence Ti_(x)O_(y)—C anode permits production of titanium particulateat a relatively high rate and relatively low cost. And current pulsingas well as periodic reversing the current can further enhance theproduction of titanium particulates when coupled with a high masstransfer rate cell as above described.

Heretofore aluminum and magnesium have been produced by a compositeanode process utilizing anodes of Al₂O₃—C or MgO—C [23-26]. However,there is no teaching a suggestion in any of the prior art thatrecognizes that high valence (4 or more) or multi-valence metals couldbe produced by a composite anode process. More importantly, it was notrecognized that high value high valence or multi-valence metals such astitanium, chromium, hafnium, molybdenum, niobium, tantalum, tungsten,vanadium, and zirconium could be produced utilizing a composite anode,as in the present invention. Neither was it recognized that a highvalence metal oxide could be thermally reduced to a lower valence statein a composite anode or that a reduced valence state metal oxide-carbonanode could be used to produce particulate metal by electroreduction.

In contrast to producing a molten metal aluminum (melting point approx.at 660° C.) and magnesium (melting point approx. at 650° C.), thepresent invention permits control of particle geometry and size, andgrain size in the particle can be controlled by the molten saltcomposition, its operating temperature and the cathode current density.Moreover, the instant invention permits direct production of metals inthe powdered/particulate solid state, unlike the prior art processeswhich produced molten aluminum [23, 25, 26] or magnesium [24].

In addition, the combination of thermal treatment to reduce the metal toa lower valence state, the use of carbon in the anode to release a lowervalence state metal into the molten salt, and the selection of moltensalt to stabilize the lower valence state metal so as to produce a fullyreduced metal at the cathode, is a unique and advantageous feature ofthe current invention.

An alternative to reducing the titanium valence in the molten salt is todepolarize the cathode using hydrogen which could not only prevent there-oxidation of the lower valence titanium at the anode and reduce thetotal cell voltage, but also allow for the formation of titaniumhydrides at the cathodes. Titanium hydride is much more stable thantitanium toward oxidation. The present invention thus permits theproduction of very low oxygen titanium.

Moreover, the present invention overcomes a problem of poor electricalconductivity of the metal oxide-carbon anode of my previous compositeanode process [23-26] which required the use of aluminum or magnesiummetal conductors through the composite anode to carry current andprevent high voltage drops due to the poor electrical conductivity ofthe Al₂O₃—C or MgO—C composite anodes. In the instant invention, pooranode electrical conductivity is overcome by using highly electricallyconductive carbon flake as the major carbon source in the compositeanode. Small size composite anode pieces can also be utilized to reducevoltage drop as illustrated in FIG. 3 as contrasted to large size anodeswhich can result in high resistivity and larger voltage drops thatincrease energy consumption. Examples of low resistivity in a reducedvalence state titanium oxide carbon anode is shown in FIG. 5. Furtherwhen the TiO₂ is reduced to TiO, the TiO is very electricallyconductive, more so than graphite. Thus anodes made with TiO are quiteconductive and in one iteration does not require pressing into acomposite with graphite flake or other carbon forms. The TiO is soconductive, it can be simply mixed with carbon/graphite in a basket thatserves as the anode with a conductor which can be the basket or agraphite rod.

The following non-limiting Examples will further demonstrate the presentinvention.

EXAMPLE 1

Titanium dioxide (TiO₂) with a purity of 99% in a particle size of 0.3microns was mixed with graphite flake in a particle size of 40 micronsin a ratio of 80 grams of TiO₂ and 20 grams of graphite flake using aball mill mixer. The resulting TiO₂-graphite flake mixture was pressedin a steel die at 50,000 psi, which provided a mechanically stablebillet without any binder system. The TiO₂-graphite flake billet washeated to 1100° C. in the absence of air for two hours. An XRD analysisshowed the resulting composite anode to consist of Ti₂O₃, Ti₃O₅ andTi₄O₇ and graphite. The resulting titanium oxide-graphite compositeanode was cut into one inch (2.54 cm) long segments, and the segmentsplaced in a carbon-carbon composite basket as illustrated in FIG. 3which had residual porosity that served as a membrane and to which thepositive terminal of a dc power supply was connected. A steel walledcontainer (illustrated in FIG. 3) was used to melt an electrolyteconsisting of NaCl—CaCl₂—KCl eutectic at a temperature of 650° C. Thesteel walled container was connected to the negative terminal of the dcpower supply. The steel walled container was covered, sealed and sweptwith an argon purge to remove any air from the system. Electrolysis wasconducted at an anode and cathode current density of 0.5 amps/cm², whichproduced titanium particulate at the steel cathode. The titaniumparticulate was harvested with a screen scoop and then subjected to1200° C. in a vacuum to remove all traces of the electrolyte. Theparticle size was in the range of one to ten microns with a predominanceof 5-10 microns. The titanium powder was analyzed for oxygen and foundto have 800 parts per million. The current efficiency was measured bycalculating the amphere hours passed and weighing the titanium producedwhich was found to be 95% at the cathode and 99% at the anode.

EXAMPLE 2

A mixture of TiO₂ and graphite flake was mixed as described in Example 1and a resin binder of phenolic was used to bind the particles in thepressing operation. The pressed body was then heated in an inertatmosphere to 1300° C., which produced a well-bonded strong compositeanode consisting of a mixture of Ti₂O₃ with some Ti₃O₅ and a smallamount of TiC along with graphite. Electrolysis was conducted as inExample 1 at a cathode current density of 1.0 amp/cm². Titanium powderwas produced at an efficiency of 90% in an average particle size of 10microns.

EXAMPLE 3

Example 2 was repeated with electrolysis at a cathode current density of0.25 amps/cm² which produced an efficiency of 97% with a particle sizeof approximately 20 microns.

EXAMPLE 4

A composite anode was produced using a mixture of TiO₂, Al₂O₃ and V₂O₃in an elemental ratio of Ti-6Al-4V. A stoichiometric ratio of graphiteflake was mixed with the oxides and a coal tar pitch binder was used.The molded composite anode was heat treated to 1200° C. in an inertatmosphere. The composite anode was placed in the anode basket asdescribed in Example 1 but a sheet of titanium was used as the cathode.The electrolyte consisted of NaCl—LiCl—KCl eutectic with 20 mole % LiF.Electrolysis was conducted at a cathode current density of 1.25amps/cm², which produced particles in a size primarily in the range of10-80 microns. The harvested particles were analyzed and found tocontain a ratio of Ti-6Al-4V.

EXAMPLE 5

A composite anode was prepared as described in Example 1 and heattreated to 1150° C. The molten salt electrolyte consisted of KF—NaF—LiFeutectic operated at 650° C. The cathode was nickel metal withelectrolysis conducted at a cathode current density of 0.25 amps/cm². Acoherent film of titanium 10 microns thick was deposited on the nickelcathode.

EXAMPLE 6

A composite anode was produced as described in Example 2 using Y₂O₃ andgraphite flake in stoichiometric ratio. The anode was electrolyzed as inExample 2, which produced yttrium metal in a particle size of 10-30microns.

EXAMPLE 7

A composite anode was produced as described in Example 2 usingstoichiometric ratio of HfO₂ and carbon. Electrolysis of the anode in amolten salt electrolyte, as in Example 4, at a cathode current densityof 0.5 ampheres/cm² produced metal hafnium metal particularate having aparticle size of 10-100 microns.

EXAMPLE 8

A composite anode was produced by mixing a stoichiometric amount ofCr₂O₃—C and a pitch binder. The composite anode was heated in theabsence of air to 1400° C. and then electrolyzed at a cathode currentdensity of 0.25 amps/cm² in a molten salt electrolyte as in Example 4. Achromium particulate was produced having a particle size of 5-40microns.

EXAMPLE 9

Rutile ore which contained approximately 95% TiO₂ was dried and mixedwith graphite flake and a resin binder to produce the oxide-carbon instoichiometric ratio. The mixture was compressed to 20,000 psi and heattreated in an inert atmosphere to 1200° C. The anode was electrolyzed asin Example 4, which produced a powder at the cathode containingprimarily titanium, and small amounts of iron, aluminum, niobium,vanadium and silicon having a particle size of 1-80 microns.

EXAMPLE 10

A salt composition of (65AlCl₃-35NaCl mole %)-20 mole % NaF was utilizedas the electrolyte at an operating temperature of 190° C. A compositeanode was utilized as described in Example 1 with electrolysis conductedwith a pulsed current 3 seconds on and 1 second off. A crystallinetitanium deposit of flake morphology was produced at a cathode currentdensity of 1 amps/cm².

EXAMPLE 11

Example 10 was repeated with a cathode current density of 0.25 amps/cm².The resulting titanium deposit was a solid film on the cathode. Thepulse scheme was then modified to 3 seconds on ¼ second off withperiodic reverse polarity and then repeating the cycle. The deposit wasa solid film with a very fine grain microstructure. Other shape formpulses provided similar results.

EXAMPLE 12

Hydrogen was used at the cathode in an electrolytic cell similar toExample 10 with or without a pulsed current. Cell voltage was decreasedby about 10 to 15%, and titanium hydride powder formed in-situ in thecell instead of metallic titanium powder. Washing the titanium hydrideproduced oxygen pick up of ≦200 ppm. The resulting titanium hydride wasthen dehydrogenated by heating to about 650° C. to produce metallictitanium powder with ≦400 ppm oxygen. This oxygen level is an order ofmagnitude lower than titanium powder produced by any other process.

EXAMPLE 13

Titanium oxide was mixed with a stoichiometric amount of carbon blackand heated under a reduced pressure of 0.01 atmosphere in argon to atemperature of 1450° C. which produced the titanium suboxide of Ti₂O₃with no other suboxides or contaminates such as TiC. The Ti₂O₃ was mixedwith graphite flake, a binder of phenobic resin, and pressed into ablock. The block was heated in the absence of air to 1100° C. whichformed an anode. The resulting composite anode was used in a fused saltconsisting of the eutectic of LiCl—KCl operated at 500° C. Electrolysiswas conducted in trial one at 1 amp/cm² on the cathode which producedtitanium particularate in a size of 1 to 10 microns. In a second trial atitanium sponge was placed in the bottom of the fused salt and TiCl₄ wasbubbled onto the sponge which produced TiCl₂ in the salt bath. TiCl₄continued until a concentration of 5% TiCl₂ was generated. Electrolysiswas then performed as in trial one and titanium particularate with asize up to 400 microns was produced, thus showing with a titanium ion insolution larger size titanium particularate was produced.

EXAMPLE 14

An identical system as in Example 13 was created before and TiCl₂ wasgenerated, and in trial one, the electrolysis was performed at 40amps/cm². The titanium particularate produced was in a size range of 20to 100 microns. In trial two, electrolysis was performed at 125 amps/cm²which produced titanium particles in approximately the same size as the40 amps/cm² current density trial. In trial three electrolysis was alsoperformed at 125 amps/cm² with argon gas bubbling over the cathode tocreate a large mass flow. The titanium particularate produced in thehigh mass flow at 125 amps/cm² was in the size range of 40 to 200microns. The titanium suboxide-carbon composite anode provides theopportunity to operate at very high cathode current densities and in ahigh mass flow regime.

EXAMPLE 15

TiO₂ and carbon were heated under a pressure of 0.01 residual argonatmosphere to 1850° C. which produced TiO and CO. The TiO was mixed withstoichiometric carbon and a binder and molded into a block which washeated to 1100° C. which formed a composite anode. The resultingcomposite anode was placed in a salt mixture of 60NaCl-40MgCl₂ and 20mole percent NaF based on the chloride salt mixture operated at 600° C.In trial one, the electrolysis was performed at 0.15 amps/cm² andtitanium particularate in the size range of 50 to 300 microns wasproduced. In trial two, a titanium sponge was placed in a small crucibleimmersed in the salt bath and TiCl₄ was bubbled onto the sponge thatproduced TiCl₂ until the concentration was 8% TiCl₂ in the salt.Electrolysis was performed at 0.15 amps/cm² which produced titaniumparticularate in the 200 to 500 micron size. The oxygen content was 380parts per million.

EXAMPLE 16

Rutile with a composition as follows, and the remainder titanium wasprocessed as shown in the headings:

Purity of After heating to Electrolytically As received 1700° C. withproduced Impurities Units composition carbon titanium Al ppm 5300 4200700 Ca ppm 570 530 <100 Cr ppm 300 150 100 Fe ppm 4390 140 100 Mg ppm1470 1270 500 Si ppm 12000 <100 <100 V ppm 2290 2290 2000 Zr ppm 360 250300 With the remainder titaniumThe rutile was mixed with carbon in a ratio of 1.1 to stoichiometry andheated to 1700° C. in argon at atmospheric pressure. The compositionafter heating is shown in the second column which shows the rutile waspurified and particularly in the case of iron and silicon of which thelatter is most undesirable as an impurity in titanium metal.

The purified rutile was mixed with carbon and resin and molded onto ablock which was heat treated to 1250° C. The composite block wasutilized as an anode in a salt bath of NaCl—CaCl₂ operated at 650° C.Electrolysis was performed at 0.5 amps/cm² which produced particularatein the size range of 50-350 microns with a purity as shown in columnfive above. Aluminum and vanadium are desirable alloying elements fortitanium and are used in most titanium alloys. Thus a relatively puretitanium is produced from low cost domestic source rutile which can meetvirtually all market demands except the stringent aerospacerequirements.

EXAMPLE 17

TiO₂ was mixed with carbon and heated in a 90% nitrogen 10% hydrogenatmosphere to 1600° C. which produced titanium nitride (TiN). The TiNwas pressed and sintered at 2000° C. in a nitrogen atmosphere. The TiNblock was used as an anode in a salt mixture of (NaCl—KCl)-20 mole % NaFoperated at 700° C. Electrolysis was conducted at 0.5 amps/cm² whichproduced titanium particularate in the size range of 20 to 350 micronsand nitrogen gas was given off at the anode.

EXAMPLE 18

TiO₂ was mixed with carbon in a ratio of 1 to 1.5 over stoichiometry andheated in argon at 1600° C. which produced titanium carbide (TiC). TheTiC was pressed and sintered at 2000° C. The TiC was used as an anode inthe same salt as in Example 17. During electrolysis at 1 amp/cm² oxygenwas bubbled under the TiC anode in an amount equivalent to the currentto produce titanium so that the oxygen reacted with the freed carbon toproduce CO₂/CO which is often referred to as depolarizing the electrode.A diaphragm of porous alumina was placed between the anode and cathodeto prevent any oxygen from contacting the deposited titaniumparticularate and oxidizing it. The particle size of titaniumparticularate produced was in the size range of 20 to 200 microns.

EXAMPLE 19

The suboxide TiO was produced by reacting TiO₂ with stoichiometriccarbon in a vacuum of 0.01 atmosphere at a temperature of 1850° C. TheTiO was then pressed and practically sintered at 1450° C. to provide aporous body which served as a cathode in a fused salt bath of calciumchloride containing 5% calcium oxide operated at 900° C. A graphiteanode was utilized and electrolysis performed at a constant voltage of3.0V for a period of 12 hours. The TiO was reduced to titanium metalwith oxygen being attracted to the anode to produce CO₂/CO.

EXAMPLE 20

Example 19 was repeated using Ti₂O₃ as the starting material.

EXAMPLE 21

Example 19 was repeated with the exception the electrolyte was theeutectic of CaCl₂—NaCl which was operated at 750° C. With the suboxideTiO, the lower temperature salt bath can be used to reduce TiO totitanium metal.

EXAMPLE 22

The molten salt bath electrolyte of CaCl₂ operated at 900° C. showed aconsiderable solubility of the reduced suboxide of titanium TiO. In asalt bath operated at 900° C. 5 wt % TiO was added and electrolysisconducted with a carbon anode. Titanium particularate was deposited onthe cathode at a current density of 1 amp/cm². In a second trial, aporous alumina diaphragm was used around the anode to prevent any oxygenfrom diffusing to the deposited titanium on the cathode andcontaminating the deposited titanium particularate.

EXAMPLE 23

A molten salt composition consisting of the CaCl₂—NaCl eutecticcontaining 20 mole % NaF was operated at 750° C. and 2 wt % TiO wasadded which became soluble in the salt bath. A carbon anode was used andelectrolysis performed at a cathode current density of 0.25 amps/cm².Titanium particularate was deposited on the cathode and CO₂/CO wasevolved from the carbon anode.

EXAMPLE 24

TiO was produced as described in Example 15 and mixed with carbonparticularate. The mixture of TiO—C was placed in a porous carbon-carbonbasket which served as the anode electrical conductor. The anode basketcontaining TiO—C was placed in a salt of LiCl—KCl eutectic containing 20wt % NaF operated at 680° C. Electrolysis was performed at 1 amps/cm²which produced titanium particularate in the size range of 50-500microns which demonstrated a physical mixture of TiO—C can serve as ananode.

EXAMPLE 25

An anode produced as described in Example 13 was utilized in theelectrolyte given in Example 13 with electrolysis conducted at 1amps/cm² concurrent with hydrogen bubbling under the cathode. Thedeposit was titanium particularate in the size range of 50-800 microns.Heating the deposit showed hydrogen evolution as detected in a massspectrometer.

EXAMPLE 26

A graphite crucible was set inside a steel cell with a cover and seal toprovide an inert atmosphere with an argon purge. A graphite rod with areduced tip to serve as a resistor was placed through a standardfeed-through in the cell cover. Calcium fluoride was placed in thegraphite crucible. The graphite rod was heated resistively between aconnection to it and the steel cell which raised the temperature to1700° C. which melted the calcium fluoride. TiO was then added at 5 wt%. Electrolysis was conducted at 1 amps/cm² between a separate graphiteanode and the crucible serving as the cathode. After six hours ofelectrolysis the experiment was stopped and the system cooled. Breakingthe salt revealed beads of titanium that had been produced in the moltensalt.

The above embodiments and examples are given to illustrate the scope andspirit of the instant invention. These embodiments and examples arewithin the contemplation of the present invention. Therefore, thepresent invention should be limited only by the appended claims.

1. A method for the production of titanium metal which compriseselectrochemically dissolving, in a molten salt electrolyte, an anodeformed of a titanium suboxide/carbon composite, and reducing thedissolved titanium suboxide, at a cathode, to titanium metal.
 2. Themethod of claim 1, wherein said molten salt electrolyte comprises astrong Lewis acid.
 3. The method of claim 2, wherein the electrolyte isselected from the group consisting of an eutectic of sodium chloride,lithium chloride and potassium chloride, an eutectic of potassiumfluoride, sodium fluoride and lithium fluoride, an eutectic of sodiumchloride, calcium chloride and potassium chloride, an eutectic of sodiumchloride, magnesium chloride and sodium fluoride, and an eutectic ofsodium chloride, potassium chloride and sodium fluoride.
 4. A method forthe production of purified titanium from rutile ore which compriseselectrowinning from an anode formed of a mixture of titaniumsuboxide/carbon composite in a molten salt electrolyte, and depositingpurified titanium onto a cathode.
 5. The method of claim 4, wherein themolten salt electrolyte is selected from the group consisting of aneutectic of sodium chloride, lithium chloride and potassium chloride, aneutectic of potassium fluoride, sodium fluoride and lithium fluoride, aneutectic of sodium chloride, calcium chloride and potassium chloride, aneutectic of sodium chloride, magnesium chloride and sodium fluoride, andan eutectic of sodium chloride, potassium chloride and sodium fluoride.6. The method of claim 4, wherein titanium suboxide is mixed with carbonin a ratio of at least 1:1.5 over stoichiometry to produce TiC andCO₂/CO.
 7. The method of claim 4, wherein the titanium suboxide is mixedwith carbon in a ratio of at least 1:1 over stoichiometry to produce TiCand CO₂/CO.
 8. A method for the production of purified titanium whichcomprises electrochemically dissolving an anode formed of a titaniumsuboxide/carbon composite in a molten salt electrolyte, andelectrochemically reducing the dissolved titanium suboxide to purifiedtitanium metal.
 9. The method of claim 8, wherein the molten saltelectrolyte is selected from the group consisting of an eutectic ofsodium chloride, lithium chloride and potassium chloride, an eutectic ofpotassium fluoride, sodium fluoride and lithium fluoride, an eutectic ofsodium chloride, calcium chloride and potassium chloride, an eutectic ofsodium chloride, magnesium chloride and sodium fluoride, and an eutecticof sodium chloride, potassium chloride and sodium fluoride.
 10. Themethod of claim 8, wherein titanium suboxide is mixed with carbon in aratio of at least 1:1.5 over stoichiometry based on titanium to produceTiC and CO₂/CO.
 11. The method of claim 8, wherein the titanium suboxideis mixed with carbon in a ratio of at least 1:1 over stoichiometry basedon titanium to produce TiC and CO₂/CO.
 12. The method of claim 8,wherein titanium suboxide-carbon composite anode is formed by heating atitanium oxide with carbon under an inert atmosphere.
 13. The methodaccording to claim 8, wherein the anode comprises a composite oftitanium suboxide and carbon, and including the step of adding a Ti⁺²containing compound to the electrolyte.
 14. A method for the directproduction of titanium metal in a particulate state which compriseselectrochemically dissolving an anode, formed of a titaniumsuboxide/carbon composite, in a molten salt electrolyte in anelectrochemical cell, and electrochemically reducing the dissolvedtitanium suboxide to particulate titanium metal.
 15. The method of claim14, wherein said molten salt electrolyte comprises a strong Lewis acid.16. The method of claim 15, wherein the electrolyte is selected from thegroup consisting of an eutectic of sodium chloride, lithium chloride andpotassium chloride, an eutectic of potassium fluoride, sodium fluorideand lithium fluoride, an eutectic of sodium chloride, calcium chlorideand potassium chloride, an eutectic of sodium chloride, magnesiumchloride and sodium fluoride, and an eutectic of sodium chloride,potassium chloride and sodium fluoride.
 17. The method of claim 14,wherein the electrolyte includes a Ti⁺³ containing compound which isreduced in one step to titanium metal.
 18. The method of claim 17,wherein the Ti⁺³ containing compound is added in a concentration of ½ to20% by weight of the electrolyte.
 19. The method of claim 18, whereinthe Ti⁺³ containing compound is added in a concentration of 1 to 10% byweight of the electrolyte.
 20. The method according to claim 14, whereinthe electrode is formed of a titanium oxide/carbon composite, andincluding the step of adding a Ti⁺² containing compound to theelectrolyte.
 21. The method of claim 20, wherein the Ti⁺² containingcompound is added in a concentration of ½ to 20% by weight of theelectrolyte.
 22. The method of claim 21, wherein the Ti⁺² containingcompound is added in a concentration of 1 to 10% by weight of theelectrolyte.
 23. A method for the production of titanium metal whichcomprises electrochemically dissolving, in a molten salt electrolyte, ananode formed of a titanium oxide/carbon composite, wherein the moltensalt electrolyte comprises a strong Lewis acid, and electrochemicallyreducing the dissolved titanium oxide to titanium metal at a cathode.24. The method of claim 23, wherein the electrolyte is selected from thegroup consisting of an eutectic of sodium chloride, lithium chloride andpotassium chloride, an eutectic of potassium fluoride, sodium fluorideand lithium fluoride, an eutectic of sodium chloride, calcium chlorideand potassium chloride, an eutectic of sodium chloride, magnesiumchloride and sodium fluoride, and an eutectic of sodium chloride,potassium chloride and sodium fluoride.
 25. A method for the directproduction of titanium metal in a particulate state which compriseselectrochemically dissolving an anode, formed of a titanium oxide/carboncomposite, a molten salt electrolyte in an electrochemical cell, whereinthe molten salt electrolyte comprises a strong Lewis acid, andelectrochemically reducing the dissolved titanium oxide to titaniummetal.
 26. The method of claim 25, wherein the electrolyte is selectedfrom the group consisting of an eutectic of sodium chloride, lithiumchloride and potassium chloride, an eutectic of potassium fluoride,sodium fluoride and lithium fluoride, an eutectic of sodium chloride,calcium chloride and potassium chloride, an eutectic of sodium chloride,magnesium chloride and sodium fluoride, and an eutectic of sodiumchloride, potassium chloride and sodium fluoride.
 27. The methodaccording to claim 25, wherein the electrode is formed of a titaniumoxide/carbon composite, and including the step of adding a Ti⁺²containing compound to the electrolyte.
 28. The method of claim 27,wherein the Ti⁺² containing compound is added in a concentration of ½ to20% by weight of the electrolyte.
 29. The method of claim 28, whereinthe Ti⁺² containing compound is added in a concentration of 1 to 10% byweight of the electrolyte.
 30. The method of claim 25, wherein theelectrolyte includes Ti⁺³ containing compound which is reduced in onestep to titanium metal.
 31. The method of claim 30, wherein the Ti⁺³containing compound is added in a concentration of ½ to 20% by weight ofelectrolyte.
 32. The method of claim 31, wherein the Ti⁺³ containingcompound is added in a concentration of 1 to 10% by weight of theelectrolyte.